1. Field of the Invention
The present invention relates to a semiconductor device and a display device. Note that in this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. Examples of such a semiconductor element include a transistor (e.g., a thin film transistor). Furthermore, a display device such as a liquid crystal display device is also included in the category of the semiconductor device.
2. Description of the Related Art
In recent years, display devices using liquid crystal panels or organic EL panels have been under active development. These display devices are broadly classified into display devices in which only transistors for pixel control (pixel transistors) are formed over a substrate and a scanning circuit (driver circuit) is included in a peripheral IC and display devices in which a scanning circuit is formed over the same substrate as pixel transistors.
A display device in which a driver circuit is integrated with pixel transistors is effective in reducing the frame width of the display device or the cost of the peripheral IC. However, transistors used in the driver circuit are required to have better electrical characteristics (e.g., field-effect mobility (μFE) or threshold voltage) than the pixel transistors.
A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor material has been attracting attention (for example, Patent Documents 1 and 2). For example, attention has been drawn to a transistor including a semiconductor thin film formed using an amorphous oxide that contains indium (In), gallium (Ga), and zinc (Zn) and has an electron carrier concentration of less than 1018/cm3.
A transistor using an oxide semiconductor for a semiconductor layer has higher field-effect mobility than a transistor using for a semiconductor layer amorphous silicon which is a silicon-based semiconductor material. Hence, the transistor using an oxide semiconductor operates at high speed and is suitably used for the display device in which a driver circuit is integrated with pixel transistors. Besides, manufacturing steps of the transistor using an oxide semiconductor are easier than those of a transistor using polycrystalline silicon for a semiconductor layer.
In the case where a transistor is manufactured using an oxide semiconductor for a channel formation region, an oxygen vacancy which is formed in the oxide semiconductor adversely affects the transistor characteristics. The oxygen vacancy functions as a carrier source in the oxide semiconductor. Therefore, a large number of oxygen vacancies in the oxide semiconductor lead to generation of electrons serving as carriers, whereby the channel formation region has n-type conductivity and thus has reduced resistance. This induces degraded electrical characteristics of the transistor, such as normally-on characteristics, an increase in leakage current, and a shift of a threshold voltage by application of stress. Hence, the oxide semiconductor preferably includes as few oxygen vacancies as possible in the channel formation region.